The use of diodes as microwave switches is well known in the art and they offer a number of advantages over mechanical switches. There are no moveable switch contacts with the attendant problem of noise due to poor connections. Diode switches can be fabricated at a lower cost than comparable mechanical switches. Diode switches can also be packaged in a smaller volume and, in particular, they are readily incorporated into transmission lines used to convey high frequency energy. Also, they are easily controlled from remote locations since the switching action is affected by means of switching and dc bias potentials applied to the switching diodes. Diode microwave switches typically use p-i-n diodes, well-known in the art, that are connected in shunt and series configurations to provide single-pole single-throw (SPST) and single-pole double-throw (SPDT) switches.
The radio frequency power that can be controlled by a p-i-n diode switch is determined by many factors well-known in the art. Several of these factors are: the frequency of the microwave energy passing through the diode, the speed at which the diodes are switched between their conducting and nonconducting states, the frequency at which the diodes are switched alternately between the conducting and nonconducting states, and the isolation level between the input and the outputs of a switch. These factors and the others not enumerated, coupled with a shunt configuration of the p-i-n diodes for maximum heat dissipation and fast switching speed, have resulted in microwave switches capable only of switching microwave power up to twenty watts of microwave energy.
In a typical p-i-n diode microwave switch isolation in the order of sixty decibels (db) between the input and output of the switch when the switch is open is required. In the art it is well-known that a single diode can typically provide in the order of 25 db isolation. Accordingly, multiple diodes are used to achieve the desired isolation, and typically three diodes are used to achieve sixty db isolation. As mentioned previously, diodes are connected in shunt across the transmission line conductors through the switch in order to achieve maximum power handling capability while achieving fast switch action. When such a switch is in its open state all the shunt connected diodes are placed in their conducting state in a manner well-known in the art. Vice versa, when such a switch is in a closed state the shunt connected diodes are caused not to conduct. Microwave energy input to a shunt connected switch in its open state divides and passes through the conducting multiple diodes to be reflected back to the input but the diodes do not conduct equal amounts of microwave energy. The one of the multiple shunt connected diodes upon which the input microwave energy is first incident conducts the largest portion of the microwave energy. As a result, this first diode heats up and, in turn, the resistance of the diode increases and the diode conducts more microwave energy until a stable state is reached. In some instances the first diode may even burn out depending upon the amount of microwave energy input to the switch. The overheating of the first diode also causes the isolation between the input and output of the microwave switch in the open state to decrease. As the heating of the first diode causes its voltage drop to decrease, the amount of dc bias current passing through the first diode to maintain it in its conducting state also increases resulting in a decrease in bias current passing through the remainder of the shunt connected diodes of the switch. This decrease of dc bias current passing through the remainder of the shunt connected diodes changes the operating point of these diodes and thereby causes a decrease in the isolation between the input and output of the microwave switch.
The above described loss of isolation in microwave p-i-n diode switches has long been a problem in the art resulting in prior art switches being capable of only switching under 20 watts of microwave power.
To solve the above described problem I provide novel circuitry for optimizing switching and dc bias conditions in a p-i-n diode microwave switch. With my novel circuitry isolation between the input and outputs of a microwave switch does not drop to unacceptable levels when microwave energy in excess of 20 watts is applied to the switch. More particularly, my novel circuitry isolates multiple shunt connected diodes in a microwave switch from each other so that a change in bias current flowing through one diode, particularly the first diode, does not effect the bias current flowing through other diodes in the switch. At the same time, the diodes do not appear to be isolated from each other with respect to microwave energy input to the switch.
My invention with its various advantages and features will appear more fully upon consideration of the attached drawings and the following detailed description thereof.